A field-emission cathode (or field emitter) emits electrons upon being subjected to an electric field of sufficient strength. The electric field is produced by applying a suitable voltage between the cathode and an electrode, typically referred to as the anode or gate electrode, situated a short distance away from the cathode.
When a field-emission cathode is utilized in a flat-panel CRT display, electron emission from the cathode occurs across a sizable area. The electron-emitting area is commonly divided into a two-dimensional array of electron-emitting portions, each situated across from a corresponding light-emitting portion to form part or all of a picture element (pixel). The electrons emitted by each electron-emitting portion strike the corresponding light-emitting portion and cause it to emit visible light.
It is generally desirable that the illumination be uniform (constant) across the area of each light-emitting portion. One method for achieving uniform illumination is to arrange for electrons to be emitted uniformly across the area of the corresponding electron-emitting portion. This typically involves fabricating the electron-emitting portion as a large number of small, closely spaced electron-emissive elements.
Various techniques have been investigated for manufacturing electron-emitting devices that contain small, closely spaced electron-emissive elements. Spindt et al, "Research in Micron-Sized Field-Emission Tubes," IEEE Conf. Rec. 1966 Eighth Conf. Tube Techniques, Sep. 20, 1966, pp. 143-147, describes how small randomly distributed spherical particles are employed to define the locations for conical electron-emissive elements in a flat field-emission cathode. The size of the spherical particles strongly controls the base diameter of the conical electron-emissive elements.
FIGS. 1a-1g (collectively "FIG. 1") illustrate the sphere-based process utilized in Spindt et al to fabricate an electron-emitting diode having a thick anode. In FIG. 1a, the starting point is sapphire substrate 20. A sandwich consisting of lower molybdenum layer 22, insulating layer 24, and upper molybdenum layer 26 is situated on substrate 20.
Polystyrene spheres 28, one of which is shown in FIG. 1b, are scattered across the top of molybdenum layer 26. "Resist" is deposited to form resist layer 30A on the uncovered part of layer 26. See FIG. 1c. Portions 30B of the resist, typically alumina (aluminum oxide), accumulate on spherical particles 28 during the resist deposition. Spheres 28 are subsequently removed, thereby removing resist portions 30B. Referring to FIG. 1d, openings 32 extend through resist layer 30A at the locations of removed spheres 28.
The exposed portions of molybdenum layer 26 are etched through resist openings 32 to form openings 34 through molybdenum 26, the remainder of which is indicated as item 26A in FIG. 1e. Similarly, the exposed parts of insulating layer 24 are etched through openings 34 to form cavities 36 through remaining insulating layer 24A. See FIG. 1f. Resist layer 30A is removed, typically during the cavity etch.
Finally, molybdenum is evaporatively deposited on top of the structure and into cavities 36. The evaporation is performed in such a way that the openings through which the molybdenum accumulates in cavities 36 progressively close. As indicated in FIG. 1g, conical molybdenum electron-emissive elements 38A are formed in cavities 36, while continuous molybdenum layer 38B is formed on top of molybdenum layer 26A. Layers 38B and 26A together form the anode for the diode.
Utilization of spherical particles to establish the locations, and base dimensions, of electron-emissive elements in Spindt et al is a creative approach to creating an electron-emitting device. However, the electrons emitted by elements 38A are collected by anode 26A/38B and thus are not utilized to directly activate light-emitting areas. It would be desirable to employ spherical particles to define the locations for small, closely spaced electron-emissive elements that emit electrons which can be utilized to directly activate light-emissive elements in a flat-panel device in a highly uniform manner.